There are two parts involved in writing - the creative bit, when you're doing it, and then the editing afterwards.
If the editing part of your brain is around when you're being creative, then you'll never finish anything, because your internal editor will keep telling you that it's not good enough. And if you've ever taught, you'll know that the best way to stop anything interesting happening is to critique too early. Get it down, then improve it later.
Of course, later, when you're editing it, the last thing you want is the creative bit going "But wouldn't a poem about puppies be better? Why not write that instead?"
Ideally, you'd have a trusted reader or two to help you analyse the work, see where the problems are, and then you can put your work into fixing them.
But the question's about self-editing.
The main trick for self-editing is to get some distance between yourself and the work. The standard way of doing this is to leave it for a while, at least a week and ideally a month. This is often referred to as "putting it in the bottom drawer for a while" or for a metaphor to the metaphor (and my personal favourite, being about both writing and whisky) "maturing it in wood".
Use a separate area of your house to edit if you can, especially in the later stages. You want to divorce the creative part from the editing part. I'll often do the first couple of edit passes at the computer, just to get rid of the truly shitty bits, but I don't think editing really starts until you get to pen and paper.
If you compose at a computer (as most people do) then you want to look at it in a different format. So print the thing out. Get a pen. Go somewhere else (the kitchen table works for me.) And start marking it up. Because you're using a different medium for editing rather than composing, the corrections you need to make will be easier to spot.
The first corrections will be huge ones. Deleting old scenes. Writing whole new ones. This is the part I love most, because it's the easiest. Just crossing a big line through the scenes which are rubbish takes a few seconds and noticeably improves the whole piece. You'll never have such a big improvement in quality for so little effort again, so enjoy this. Write corrections and new scenes in the margin, or new pieces of paper. Concentrate on the bigger picture. Then take your marked-up papers to the computer and type them in.
Then do it again.
Then do it again.
Then do it again.
And keep going.
Every time, you'll find that the corrections you need to make are smaller. Each time you edit, you'll see that there's less to fix. Keep doing it, and eventually you'll get to the point where you're just changing individual words and correcting typos.
Keep going until your changes aren't making it better - just different. You'll know when that is.
The next few bits of advice are mostly for screenwriting, but should also be useful for novelists.
Do a character pass - look at each scene from the point of view of each character in it. If I were that character at that point in the scene, what would I know? What would I feel? What would I do and say? And if they're not doing, feeling, saying that thing, correct it, type it up, and move on to the next character in the scene. And when the scene has been done from all points of view, read and mark up from the point of view of the omniscient narrator. And when that's done, move on to the next scene.
Do a dialogue pass - read it aloud. If you're working on a play (radio, screen, or TV) rather than a novel, this is best done with others - actors, if you know some, will usually this for the price of a pint or some wine and a cheap dinner. You can make pasta-and-pesto for 15 people and still have change out of a tenner. In order of preference to do this: trained actors, amateur actors, your friends, the mechanical voice on your computer, yourself. This helps you get a handle on whether the dialogue feels real or not. If it's stumbling on someone's tongue, you'll know.
Do a spelling pass - don't concentrate on what the words mean, just whether they're spelled correctly. You'll still miss some. I've heard that this can be done more easily by reading backwards, so you can't actually parse the sentences, but I've never actually tried it that way.
I've only got one external link for you, but as far as I'm concerned it's the most useful editing link in the world.
This is a list of proofreading marks. Print it out and have it next to you while editing. No more trying to remember what you meant to change, or scribbling all over the sheet - it's a series of standardised marks to help you edit. Which means you can do it faster.